1. Field of the Invention
This invention relates to an electrical apparatus for detecting relationships in three phase AC networks and more particularly to an apparatus for determining phase sequence and detecting presence of all three phases in AC networks.
2. Background Art
The correct phase sequence is very important for three phase devices connected to an AC network because it determines the direction of rotation of asynchronous motors. Incorrect phase sequence would cause a backward rotation of such motors.
It may also happen, e.g., in complicated switchboards that the same phase can be connected twice instead of the third phase or even the zero wire can be connected instead of one phase.
Another important thing is to determine voltage differences between the three phases.
So, it is very important to determine the phase sequence, the presence of all three phases and their magnitude when different devices, e.g., main and subsidiary switchboards, motors, etc., are connected to an AC network. Measuring of these relationships are determined, in most cases, in the field. An apparatus for measuring or determining these relationships should be light, easy to carry and, last but not least, inexpensive.
Various devices are known for determining the correct phase sequence, presence of all three phases and voltage differences. Control devices for determining phase sequence contain small asynchronous motors, control devices with different lamps and induction measuring instruments, e.g., voltmeters.
The fundamental disadvantage of many of the above mentioned devices is caused by their large dimensions, resulting in only limited use in the field. Devices containing neon lamps have only a limited range of measurable voltages.
A device for indicating reverse phase sequence or single phase is described, for example, in U.S. Pat. No. 3,596,137 issued Jul. 27, 1971 which uses a silicon controlled rectifier (SCR) connected by its main current path between at least two phases of a measured polyphase network. In the case of a correct phase sequence, the SCR is switched on and the current flowing through it switches a relay. In the case of a reverse phase sequence, the SCR is not switched on as the firing circuit does not send a firing signal to the control electrode of the SCR. The disadvantage of this device is the need of a firing circuit and a relay indicating the correct phase sequence. The device is therefore rather bulky, complicated and expensive.
Another device called "Waveform Crossing Detector" is described in U.S. Pat. No. 4,495,461 issued Jan. 22, 1985. This device detects a pair of waveforms of a three phase AC supply and includes a pair of opto-couplers driven by respective transistors acting as actual crossing detectors. The transistors are driven by different phase sequences of a three phase network. Final signals are processed in logical circuits. The disadvantage of such device is that a stabilized power supply for feeding the logical circuits is necessary, causing large dimensions and higher price.
Another device for testing multiphase outlet with visual indication of phase energization, phase rotation and supply voltage is known from U.S. Pat. No. 4,885,532 issued Dec. 5, 1989. This device is intended for continuous measuring of an AC network as used in explosive and inflammable areas, e.g., in underground mines. The device, according to this invention, uses neon lamps indicating presence of each phase of a three phase AC system and the phase rotation direction. The device typically includes means to alter the safe handling voltage level from AC to DC and a voltmeter into which this direct current is applied. The voltage between phases is indicated by the voltmeter. As already mentioned above, this device is usually intended for purposes of continuous stationary measurements and not for measurements when a new or repaired AC device is connected to an AC three phase network where compact and small measuring apparatus are desired.